I initially missed the issue at the base of the tree. It looks like a root (or roots) died beneath the dead part. What happened?
Some trees have very direct connections between branches and roots such that when one or the other dies, its above or below mate also dies. Junipers and yews are examples of trees that have very strong individual root to branch connections.
The center has rotted out of this old yew ^^^, leaving the veins standing in a circle and feeding individual branches.
Live vein on a juniper bonsai:
Other species of trees show a measure of the trait that varies by species. In the case of your tree, it looks like a root died and the cambium just above died as well. In maples, Water and nutrients moving upward through xylem tissues and photosynthate (food) moving downward are somewhat able to move sideways/diagonally such that a root on the south side of the trunk is able to serve a branch on the north side of the tree, but it can't simply move resources in an immediately circumferential manner. At some undetermined height it's likely the sap will eventually be able to feed the north side, but the cambium between the dead root and where resources are flowing will remain dead. Unfortunately, the tree is destined to become a future hazard.
Al
